Allow for environment variables to set settings

[ngriffiths13]

It would be really convenient to allow for the use of environment variables to define settings. Specifically I think this would be useful when deploying via Docker. I don't really care about most of the marimo settings when deploying an app because I am no longer developing, but would love to easily be able to do something like:

ENV MARIMO_THEME="dark"

[mscolnick]

This is something we want to do: bridge CLI args, .marimo.toml, and env variables into one resolved config. we just havent gotten to it yet

[ngriffiths13]

Maybe I'll take a look at it. Do you think this would be pretty straightforward?

[mscolnick]

I think it would involve coming up with a consistent / neat design that almost makes new config automatically work across all 3.
i think though we just start with 2 (env and marimo.toml).

we have the UserConfigManager which could handle all this. just thinking out-loud, but could look something like:

- abstract class MarimoConfigGetter
- TomlMarimoConfigGetter
- EnvMarimoConfigGetter
- DefaultMarimoConfigGetter
- CompositeMarimoConfigGetter([EnvMarimoConfigGetter, TomlMarimoConfigGetter, DefaultMarimoConfigGetter])